1. Field of the Invention
The present invention relates to an automatic decompression device for an engine, including a camshaft having a valve operating cam for moving a cam follower operatively coupled to an exhaust valve or an intake valve.
A decompression cam includes a ring shape surrounding the camshaft adjacent to the valve operating cam and includes a cam surface engageable with the cam follower at a position corresponding to a base circle portion of the valve operating cam and a counterweight shaped out of contact with the cam follower and disposed substantially opposite to the cam surface across the axis of the camshaft. A spring is disposed under compression between the camshaft and the decompression cam for normally urging the decompression cam in a direction to cause the cam surface to project beyond the base circle portion of the valve operating cam. The decompression cam is supported on the camshaft for movement between a biased valve opening position in which the decompression cam causes the cam surface to project beyond the base circle portion of the valve operating cam to bias open the exhaust valve or the intake valve in a compression stroke of the engine. A retracted position in which the decompression cam retracts the cam surface inwardly of the base circle portion of the valve operating cam under centrifugal forces acting on the counterweight against spring forces of the spring when the engine operates at a relatively high rotational speed.
2. Description of Background Art
An automatic decompression device is described in Japanese Patent Publication No. 3-39167.
In the conventional automatic decompression device, the ring shaped compression cam has a hole having an inside diameter slightly larger than the outside diameter of the camshaft. The camshaft is loosely fitted in the hole to allow the compression cam to move between the biased valve opened position and the retracted position. There is only a relatively small clearance defined circumferentially between the camshaft and the decompression cam. When the engine is shut off, oil tends to be trapped in the clearance, and its viscosity increases as the ambient temperature decreases. Therefore, when the engine starts at low temperatures, the decompression cam may not operate smoothly due to the viscosity resistance imposed by the oil.